It is known that in the steel industry, when considering the substantial increase experienced both in costs of row material and of the power employed, and the greater competitivity required by the global market, as well as the increasingly restrictions in the anti-pollution standards to be adopted, it is particularly felt the need of a method for manufacturing hot rolled, high quality coils and sheets, that requires lower costs of investments and production, thus giving rise to thinner and thinner thicknesses of the produced strip. A consequence thereof is that higher competitivity can be given also to the industry of transformation of the end product with lower consumptions of power, thus reducing to a minimum also the harmful impact on the environment.
Meaningful steps in this direction have been made by the technology of the last years, as shown by patents EP 0415987, 0925132, 0946316, 1011896, all in the name of the present applicant, like also the international publication WO 2004/0262497.
However, the results obtained so far, although optimal as far as the product quality is concerned (especially for the steel strips), have turned out to be improvable under the aspect of the lay-out compactness and of the energy saving, as well as of the possible enlargement of the range of flat products that can be obtained.
If in fact the so-called concept of “Cast Rolling” is for example considered, which is already present in the above-mentioned EP 0415787 in the first step of the process only and with only one rolling stand provided on the bow-shaped caster, the consequence was an intermediate product which, after a heating step, required a second rolling step.
Also in the more recent WO 2004/026497 the above-mentioned “Cast Rolling Technology” joins the continuous casting with a first rolling step, formed of not more than four stands to obtain an intermediate product that subsequently is cut and, after a heating step, is further processed with a plastic stretching and a second rolling step. According to the same publication WO 2004/026497 it is also provided the possibility of withdrawing sheets after the first roughing step, but without a controlled cooling system, as required for producing high-quality sheets. In practice the possibility of withdrawing sheets has only the function of a buffer in case of failures in the downstream process in order to avoid stops of the continuous casting and consequently of the line production, but with no relation to programmed production of sheets.
The same concept of “Cast Rolling” was also present in EP 0823294 which however provided for three distinct manufacturing steps: one first step of roughing in austenitic phase giving rise to an intermediate product; a second step of intensive heating of such an intermediate product up to temperatures <738° C., with phase transformation in the Fe/C diagram; and a third step of finishing rolling in the ferritic phase. The teaching of this prior document is substantially that of applying the concept of cast rolling to obtain a strip of thin thickness in three distinct process steps, the last of which is exclusively in the ferritic phase, thus excluding that the so-called “mass flow” (in other words the quantity of steel flowing in the time unit at the outlet of continuous casting) may be such to allow that an ultrathin product can be obtained in a single manufacturing step totally in the austenitic field.
Also patent EP 0889762 discloses how to apply the cast rolling concept for manufacturing thin strips in one single step without discontinuity and teaches how to combine the manufacturing step in continuous casting of a slab having high mass flow (thickness of the slab in meters multiplied by the outlet speed in m/min>0.487 m2/min) and a high temperature (about 1240° C.) at the outlet of the continuous casting itself, with the rolling step after a temperature homogenization step.
As already done in EP 0823294 also in EP 0889762 there is taught in fact how a cooling step or, in alternative, a heating step can be provided between the first roughing stands and the last finishing stands. Simulations and tests have made clear that the teaching of this patent cannot be applied on industrial scale. The idea of having at the continuous casting outlet a high temperature (about 1400° C.) in order to exploit as much as possible the thermal mass in the subsequent rolling step is in fact certainly interesting but not feasible in practice, because it has been found that feasible casting a slab with high mass flow, at such a high temperature that the surface temperature at the continuous casting outlet is higher than 1150° C., results in irregularities in the meniscus region, thus causing defects in the slab and more risks of break-out.